I have been trying to track down an intermittent misfire in my 2005 SV8 to no avail. My car has ~44k miles. This forum has been invaluable to helping me understand the car. I have swapped coils and confirmed the misfire doesn't travel with the coils, so I think the coils are fine.

I am beginning to suspect a dying battery (e.g., low voltage at start up causing misfire) and have a few questions I could not find answers to.

My car has sat overnight since it was last running, and my battery voltage is around 12.1 V, measured using a DMM across the two terminals. Is this too low?

When I had the car running yesterday, I measured somewhere around 14.5 Volts at idle.

Assuming my battery voltage with the car off (12.1 v) is too low, what next? Should I check for a parasitic drain? Anyway to check the health of the battery generally?

My battery appear has dates on it of 2011 (mfg date) and 2012 (install date?) and looks like a Jaguar-branded battery. Are these still available from the dealer (or elsewhere)?

Thanks.

 

What codes do you have? Assume you identified misfiring cylinders via thrown codes?

If you don't have a scanner, drive to any auto parts place and have them scan. A generic scanner will show engine codes

Have them test battery also. 12.1 VDC (fully charged) is marginal, though may not be reason for continuing misfire

Is misfire at start up only? Do you show any other engine codes?

 

For the past month or two, it has been randomly (maybe 4 times total) thrown the following:
P0305 (Cylinder 5 Misfire Detected)
P1316
The CEL illuminates, when it does, at startup. If I clear the code, it stays clear for several days.

After suspecting a failed coil on 5, I swapped it with the one on 3. Just this past week, I read the following codes:
P0300 (Random/Multiple Cylinder Misfire Detected)
P0304 (Cylinder 4 Misfire Detected)
P0305 (Cylinder 5 Misfire Detected)
P1316

Just like before, this happened on start up. I cleared the codes two days ago and they have not resurfaced. The interval between clearing codes and them getting set again is decreasing, so it seems to be getting worse.

 

Suggest check/replace plugs (original plugs? use OE NGK only)

Check for vacuum leak(s)

Check for MAF code(s)

 

...and, get battery checked, plus clean the bat cable connections

cyls 4, 5 adjacent in FO; any crank or cam sensor codes?/ check ECU history

 

I replaced the spark plug on cylinder 5 only, just to see if that is the culprit. If the codes don't reappear, I'll probably replace them all for peace of mind and to establish a baseline. No CEL after my morning commute.

I do not see any other codes (MAF or otherwise) using the code reader I have.

 

It's liable to be the coil...